Question

the same A certain telescope has an objective mirror with an aperture dlameter of 210 mm and a focal length of 1 890 mm. It captures the Image of a nebula on photographic film at its prime focus with an exposure time of 1.55 min. To produce the same light energy per unit area on the film, what is the required exposure time to photograph the same nebula with a smaller telescope that has an objective with a 50.0-mm diameter and a 900-mm focal length? 900-mm focal length min

Answer 1